The gain of conventional integrated amplifier circuits is typically influenced by changes in temperature and process variations that may occur during fabrication. For example, the gain of the bipolar emitter coupled pair (ECP) differential amplifier illustrated by FIG. 1 is typically a function of the beta (.beta.) of the bipolar transistors therein. As understood by those skilled in the art, .beta. may be influenced significantly by changes in temperature and by variations in bipolar fabrication processes. The voltage gain (A.sub.v) of the ECP differential amplifier of FIG. 1 is frequently expressed as: EQU A.sub.v =.beta.R.sub.c I.sub.e /(.beta.+1)V.sub.t (1)
where V.sub.t is the thermal voltage (V.sub.t =KT/q). Equation (1) illustrates that the voltage gain is strongly dependent on the value of .beta. for relatively small .beta..
As illustrated by FIG. 2, a common technique for limiting fluctuations in the voltage gain of an ECP differential amplifier includes the use of a compensation resistor, R.sub.comp, in a bipolar current mirror which generates the bias current I.sub.bias. The inclusion of the compensation resistor provides some degree of beta compensation by increasing the voltage on the base of Q4 and thereby increasing the collector current in Q4. However, to provide adequate compensation, the size of R.sub.comp frequently has to be relatively large, which may increase the chip area required by the differential amplifier. Compensation resistors also typically have parameters that vary with temperature and such variations can also contribute to gain error. In addition, the bipolar transistors within the current mirror of FIG. 2 can consume relatively large amounts of power.
Attempts have also been made to substitute MOS devices for the bipolar devices of FIG. 2 in order to reduce power consumption requirements in current mirrors. For example, FIG. 3 illustrates a conventional ECP differential amplifier having a MOS-based current mirror therein that generates a bias current (l.sub.bias) at a level equal to (N)(IREF), where IREF denotes a magnitude of a reference current provided by a fixed current source of conventional design and N designates the mirror gain (e.g., width of NMOS transistor M6 relative to the width of NMOS transistor M5). MOS-based current mirrors can provide additional benefits over bipolar-based current mirrors, including higher output impedance, lower compliance voltage and lower noise. MOS-based current mirrors may also require less decoupling capacitance and typically have improved scaling capability relative to bipolar-based current mirrors. Nonetheless, MOS-based current mirrors typically do not provide significant built-in compensation for .beta. variations and because of the high gate impedance of MOS transistors compensation resistors typically may not be used successfully.
Thus, notwithstanding the above-described circuits for biasing and compensating differential amplifiers, there continues to be a need for improved biasing circuits that can have low power consumption requirements and can provide excellent compensation for .beta. variations.